1. Field of the Invention
The present invention relates to a sheet feeding device provided for a computer, a copying machine, a facsimile apparatus, a word processor, a typewriter, and various kinds of other machines having a mechanism to feed sheets.
2. Related Background Art
As shown in JP-A-59-177243, hitherto, such a kind of device is constructed in a manner such that a travelling vibration wave is formed in an electric member to sandwich a sheet, thereby feeding the sheet.
The sheet conveyance principle in the above conventional example will now be described with reference to FIG. 6.
A sheet 15 is sandwiched with a moderate pressure by vibration members 13 and 14. A travelling flexural vibration (travelling vibration wave) is formed in each of the vibration members 13 and 14. A phase difference between the travelling vibration waves is spatially set to 180.degree., so that the travelling vibration waves of the vibration members 13 and 14 progress in a manner such that their convex portions always face the sheet 15 side. At this time, when an attention is paid to a certain material point of, for instance, a convex portion on the surface of each of the vibration members 13 and 14, the material point generally executes a motion which draws an elliptic locus. In FIG. 6, when considering the vibration member 13, in the case where the travelling vibration wave progresses to the right, the above material point draws a clockwise elliptic locus as shown in the drawing. Therefore, the moving direction of the material point of the convex portion of each of the vibration members 13 and 14 is opposite to the travelling direction of the vibration. Such a motion acts as a force to feed the sheet 15.
On the other hand, in the concave portion, although the sheet feeding force in the same direction as the travelling direction is generated, the pressure is smaller than that in the convex portion. Therefore, the frictional forces between the sheet 15 and the vibration members 13 and 14 are small the sheet conveying forces are also small. Thus, as a sum of the sheet feeding forces, the total force acts in the direction opposite to the travelling direction of the flexural vibration mentioned above.
FIG. 7 shows an example of an apparatus for generating a sheet conveying force as mentioned above. Reference numerals 13 and 14 denote the above vibration members. The sheet 15 is sandwiched between both of the opposite vibration members 13 and 14.
The vibration members 13 and 14 are constructed by fixedly attaching vibrators 16 and 17 onto the upper and lower surface sides of elastic members 13a and 14a formed like tracks. Each of the vibrators 16 and 17 comprises two groups of piezo-electric elements. Assuming that the values which are respectively integer times as long as a wavelength .lambda. at a degree which is used for driving are set to circumferential lengths of the elastic members 13a and 14a, both of the piezo-electric element groups are arranged so as to have a positional phase difference which is an odd number times as large as .lambda./4. By applying AC voltages having a phase difference of 90.degree. to both of the piezo-electric element groups, a travelling vibration wave as shown in FIG. 2A is formed by synthesizing the standing waves which are formed in the elastic members by both of the piezo-electric element groups.
Reference numeral 20 denotes a bottom plate of the sheet feeding device main body fixed to, for instance, a printer; 19A and 19B indicate supporting side plates provided on both sides of the width direction of the bottom plate 20; and 18 represents a press supporting member having a spring property which is arranged between the supporting side plates 19A and 19B.
The lower vibration member 14 is supported by the bottom plate 20 and the upper vibration member 13 is supported by the press supporting member 18. On the other hand, the upper vibration member 13 presses the sheet 15 with a proper force by the spring property of the press supporting member 18 and sandwiches the sheet 15 together with the lower vibration member 14. A certain frequency voltage is applied to each piezo-electric element of the vibration members 13 and 14 and vibrations are applied thereto. Thus, the sheet conveying force is generated because the convex portions of the vibration members 13 and 14 always face as mentioned above, so that the sheet is conveyed in the direction shown by an arrow in the diagram. In the diagram, although arrows indicate both directions, this means that the sheet conveying direction can be reversed by switching the vibration travelling direction.
A thickness of a corresponding rectilinear portion on one side of each of the elastic members 13a and 14a is set to be thinner than that of the other rectilinear portion so as not to come into pressure contact with the sheet, thereby preventing that the feeding force in the direction opposite to the sheet conveying direction is given to the sheet by the rectilinear portion on one side.
However, in the above conventional example, since the elastic members are formed like tracks, the sheet conveying force effectively acts only in the rectilinear portions of the elastic members. Therefore, at the end of the sheet in the conveying direction, a space which cannot be conveyed remains by only a length of the radius in the arc portion of the track. For instance, in the case where a print head 21 is provided in front of the elastic member in the sheet conveying direction, the portion corresponding to only the space which cannot be conveyed becomes an unprintable space. On the other hand, although the above space is reduced if a radius of an arc portion of the elastic member is set to a small value, it is very difficult to design the vibration member having a track shape in which the arc portion is small.